As a gas sensor for detecting concentration of oxygen or NOx in an exhaust gas discharged from an automobile or the like, there has been known a gas sensor including a sensor element which uses a solid electrolyte.
As shown in FIG. 10, this type of gas sensor is configured such that a flange portion 3a is formed on a bottomed cylindrical sensor element (oxygen sensor element in this example) 3 in the vicinity of the center of the sensor element 3, and the sensor element 3 is inserted into and held on the inside of a cylindrical housing (main body fitting) 200 (JP-A-2007-285769 (FIG. 11)). A stepped portion 200e is formed on an inner peripheral surface of the housing 200, and a cylindrical ceramic holder 5 is arranged on a proximal end side of the stepped portion 200e by way of a packing 12. By bringing the flange portion 3a of the sensor element 3 into contact with the ceramic holder 5 from a proximal end side by way of a packing (not shown in the drawing), the flange portion 3a is brought into contact with the stepped portion 200e in an indirect manner.
Further, in a gap defined between the sensor element 3 and the housing 200 in the radial direction on a proximal end side of the flange portion 3a, a cylindrical sealing material (talc powder) 6 and an insulation member (ceramic sleeve) 50 are arranged. A metal ring (flat washer) 60 is arranged on a proximal end side of the insulation member 50, a caulking portion 200a is formed by bending a proximal end portion of the housing 200 inward so that the insulation member 50 is pressed toward a distal end side and collapses the sealing material 6 by pressing whereby the whole sensor element 3 is sealed while being fixed by caulking.
Further, a cylindrical outer sleeve 400 is joined to the proximal end portion of the housing 200 so as to hold a lead line and a terminal mounted on the proximal end side of the sensor element 3 and to cover the proximal end portion of the sensor element 3. On the other hand, a distal end of the sensor element 3 is covered with a protector 7. By threadedly mounting a male threaded portion 200d of the housing 200 of the gas sensor manufactured in this manner in a threaded hole formed in an exhaust pipe or the like, the distal end of the sensor element 3 is exposed in the inside of the exhaust pipe whereby the sensor element 3 can detect a gas to be detected (exhaust gas).